From U.S. Pat. No. 4,096,203 a process for the preparation of a block copolymer composition for use in adhesive compositions is known, which process involves in the first two steps the preparation of a lithium terminated monoalkenyl arene(A)-conjugated diene(B) diblock copolymer AB--Li by means of solution anionic polymerization. In the third step the AB--Li diblock copolymer subsequently is reacted with a coupling agent in such amount and under such conditions that the coupling efficiency of the process is varied within the range of from 20 to 80%, thereby controlling the cohesive strength of the adhesive composition wherein the block copolymer composition is to be applied. Said U.S. patent discloses three possible methods for controlling, i.e. reducing, the coupling efficiency, namely
(1) underdosing of the coupling agent, i.e. adding less coupling agent than the stoichiometric amount required for complete coupling of the living block copolymers;
(2) premature addition of terminating agent; and
(3) performing the coupling reaction at elevated temperatures, so that thermal termination of a part of the living block copolymers occurs.
If the coupling agent is a difunctional coupling agent such as e.g. dibromoethane, it will be appreciated that controlling the coupling efficiency in fact means controlling the ratio between triblock and diblock copolymer in the final block copolymer composition. However, controlling this triblock/diblock copolymer ratio by the above described methods is not very precise and cannot offer the desired optimum flexibility. An important factor in this respect is the fact that every coupling agent has a maximum coupling efficiency lower than 100%. Consequently, it is only possible by any of the methods described above to accomplish a coupling efficiency which is lower than the maximum coupling efficiency of the coupling agent employed. This considerably limits the range within which the triblock/diblock copolymer ratio can vary.
Morever, commonly applied difunctional coupling agents such as dibromoethane cause the coupled product to have a relatively poor color stability.
Hence, it is an object of the present invention to provide a process for the preparation of a block copolymer composition comprising a linear vinyl aromatic-conjugated diene triblock copolymer and a vinyl aromatic-conjugated diene diblock copolymer, wherein the weight ratio between triblock and diblock copolymer may have any desired value. It is a further object to provide a process wherein a very effective and high level of controllability of the weight ratio between the tri- and diblock copolymer in the composition on the one hand and their respective molecular weights on the other hand can be achieved in order to obtain tailor made polymer compositions having the desired properties. It is also an object of the present invention to provide a process for preparing a block copolymer composition which has very good color stability.
Another obvious process for preparing a block copolymer composition comprising a triblock copolymer and a diblock copolymer, both being derived from a vinyl aromatic monomer and a conjugated diene, is physically blending the triblock and diblock copolymer, which have been prepared independently. However such process would require large blending capacity and therefore is undesired. Moreover, it will be appreciated that the separate preparation of the tri- and diblock copolymer makes it extremely difficult to accomplish that the vinyl aromatic polymer block of the diblock copolymer is substantially identical to one of the terminal vinyl aromatic polymer blocks of the triblock copolymer.
Therefore, beside the fact that the present invention aims to prepare both triblock and diblock copolymer in one process, it is also an object of the present invention to provide a block copolymer composition wherein the vinyl aromatic polymer block of the diblock copolymer is substantially identical to at least one vinyl aromatic endblock of the triblock copolymer.
From U.S. Pat. No. 4,335,221 a composition of linear triblock copolymers having a monovinyl aromatic content of 70-80% by weight and a conjugated diene content of 20-30% by weight is known, prepared by a reinitiation process comprising three process stages:
(1) polymerization of 40 to 80% by weight of the total monovinyl aromatic monomer to be used in an inert solvent in the presence of a monolithium-hydrocarbon initiator until the monomer has been substantially completely converted;
(2) adding the remaining amount of initiator, adding a further amount of 15 to 30% by weight of monovinyl aromatic monomer to be used and polymerizing until the monomer has been substantially completely converted; and finally
(3) adding a mixture of all conjugated diene monomer to be used and the remaining amount of monovinyl aromatic monomer and polymerizing until complete conversion of the monomers, followed by the addition of a protonactive compound in order to terminate the polymer chains formed.
As the rate of polymerization of a conjugated diene is in general greater than that of a monovinyl aromatic compound if both are present in one reaction mixture, it will be appreciated that in step (3) a tapered polymer block is formed. As a result, all block copolymers which are present in the polymer composition have a terminal tapered copolymer block.
European Patent Application No. 0,242,614 discloses a multistage solution polymerization process for producing a mixture of several monovinyl aromatic-conjugated diene block copolymers, each having a terminal tapered copolymer block and a monovinyl aromatic content between 55 and 95% by weight. Said process comprises five subsequent polymerization stages, whereby the polymerization reaction in each stage is allowed to proceed until substantially no free monomer is present anymore. Said five stages are:
(1) initiator and monovinyl aromatic monomer,
(2) initiator and monovinyl aromatic monomer,
(3) conjugated diene monomer,
(4) initiator and monovinyl aromatic monomer, and
(5) conjugated diene monomer and monovinyl aromatic monomer.
The polymer blends prepared according to the above described multiple initiation processes are rather hard products due to the relatively high content of monovinyl aromatic compound. Such blends are useful in products requiring good craze-resistance, little colour and a high degree of transparency.
The present invention, however, aims to provide a polymer composition of two linear block copolymers, which is elastic and which can be suitably applied in e.g. adhesive formulations and which can be prepared in one process without needing any blending equipment. It will be appreciated that this is very attractive from an economical point of view. It is a further object of the present invention to provide block copolymers having a narrow molecular weight distribution of the vinyl aromatic polymer block. In this connection it is noted that the polymer compositions of the before-mentioned prior art multiple initiation processes are described to have a polymodal molecular weight distribution, whereby the initially formed monovinyl aromatic polymer blocks have different molecular weights due to reinitiation during the formation of said monovinyl aromatic blocks.
All the before-mentioned objects have been met by the process according to the present invention.